Recently, systems that perform monitoring with wearable cameras or similar devices have been put to practical use. One of such systems detects items that are taken away or left behind on the basis of images from a wearable camera worn by a security guard. An item that is taken away (for example, stolen) is detected by identifying an item that appears in registered images but does not appear in the captured image. On the other hand, an item that is left behind (for example, planting a dangerous item, such as a bomb) is detected by identifying an item that does not appear in the registered images but appears in the captured image.
An image captured by a wearable camera varies depending on the position, orientation, and other parameters of the camera. Thus, information on the captured space (positioning information) is required. That is, a wearable camera includes sensors, such as a GPS, a gyro-sensor, and a magnetic direction sensor, which acquire positioning information serving as attribute information of a captured image. Then, an image corresponding to the positioning information is selected from the registered images (hereinafter, this image is referred to as “reference image”). In other words, a reference image capturing the same space as that in the captured image is identified by the positioning information. A comparison of the reference image and the captured image allows a varied region to be detected in the space. Based on such detection, an item taken away or left behind is identified as described above.
To detect a varied region in a space with an image captured by a portable camera and a reference image, the captured image and the reference image must first be aligned. In other words, a reference image that corresponds to the captured image must be selected.
There are two possible approaches of alignment:
An approach using sensors, such as a GPS, as mentioned above; and
An approach involving image processing such as pattern matching of images.
A technique of alignment by image processing is described in, for example, Patent Document 1.